In order to analyze the thermal diffusivity properties of a material, the thermal diffusivity can be calculated by applying one side of the material with heat and measuring the temperature at the other side. However, when the dimensions of the material is as small as on the nano scale, the direct measurement methods in the past are no longer suitable.
Currently, the methods for measuring thermal diffusivity include the flash method, the 3ω method, the AC calorimetric method, the modulated laser technique, the instantaneous phase portrait method, and the traveling wave method. Although these measurement methods are mainly used to measure thermal diffusivity of nano scale films, many problems exist herein. For example, it is very difficult to measure the actual temperature of nano scale samples.
In order to avoid the foregoing difficulty, the thermal traveling wave method of measuring the thermal diffusivity of films was issued.
The thermal traveling wave method has been developed into many different forms, and the formula represented by equation (1) is the most representative:
                              Δ          ⁢                                          ⁢          θ                =                  45          +                                    180              π                        ⁢                                                            f                  ⁢                                                                          ⁢                  π                                α                                      ⁢            d                                              (        1        )            
In the equation (1), Δθ represents the phase difference between the thermal source and the thermal traveling wave at a distance d, f represents the frequency of the thermal traveling wave or the thermal source wave, and a represents the thermal diffusivity.
Since the aforementioned thermal traveling wave method employs an optical technique to perform the measurement, the signal is very weak and the diffraction limit issue has to be considered. Furthermore, the demands on the measurement environment are extremely strict, as any slight ambient vibration affects the signal of the thermal traveling wave. Moreover, the demands on the measure sample are also strict since the film to be measured must be mirror processed in order to reflect light. Due to the optical diffraction limit, the spatial resolution cannot be less than 250 nm, and the surface layer can be measured because multiple film measurement is not possible.